KVM: x86: Check XSS validity against guest CPUIDs

Maintain per-guest valid XSS bits and check XSS validity against them
rather than against KVM capabilities. This is to prevent bits that are
supported by KVM but not supported for a guest from being set.

Opportunistically return KVM_MSR_RET_UNSUPPORTED on IA32_XSS MSR accesses
if guest CPUID doesn't enumerate X86_FEATURE_XSAVES. Since
KVM_MSR_RET_UNSUPPORTED takes care of host_initiated cases, drop the
host_initiated check.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b1e2a2e033f0c7589fc6a5be8a2219386e83e578..5865c9b77b6d280d0857b387d0c1b2d9b2f7dc26 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -815,7 +815,6 @@ struct kvm_vcpu_arch {
        bool at_instruction_boundary;
        bool tpr_access_reporting;
        bool xfd_no_write_intercept;
-       u64 ia32_xss;
        u64 microcode_version;
        u64 arch_capabilities;
        u64 perf_capabilities;
@@ -876,6 +875,8 @@ struct kvm_vcpu_arch {
 
        u64 xcr0;
        u64 guest_supported_xcr0;
+       u64 ia32_xss;
+       u64 guest_supported_xss;
 
        struct kvm_pio_request pio;
        void *pio_data;

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index efee08fad72ea9277f95ec0e24f3b4bd34e799a8..6b8b5d8b13ccbec4a537b0e507a9757b137a7139 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -263,6 +263,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
        return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
 }
 
+static u64 cpuid_get_supported_xss(struct kvm_vcpu *vcpu)
+{
+       struct kvm_cpuid_entry2 *best;
+
+       best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
+       if (!best)
+               return 0;
+
+       return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
+}
+
 static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
                                                       struct kvm_cpuid_entry2 *entry,
                                                       unsigned int x86_feature,
@@ -424,6 +435,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
        }
 
        vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu);
+       vcpu->arch.guest_supported_xss = cpuid_get_supported_xss(vcpu);
 
        vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);
 

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 757878a222a7b1348f3cbc11cee675c16ac9353d..6ae12e8c9d052a99f24ce6d716540b9e689088f6 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3984,15 +3984,14 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                }
                break;
        case MSR_IA32_XSS:
-               if (!msr_info->host_initiated &&
-                   !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
-                       return 1;
+               if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
+                       return KVM_MSR_RET_UNSUPPORTED;
                /*
                 * KVM supports exposing PT to the guest, but does not support
                 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
                 * XSAVES/XRSTORS to save/restore PT MSRs.
                 */
-               if (data & ~kvm_caps.supported_xss)
+               if (data & ~vcpu->arch.guest_supported_xss)
                        return 1;
                vcpu->arch.ia32_xss = data;
                vcpu->arch.cpuid_dynamic_bits_dirty = true;